The invention relates to a circuit for measuring the impedance of an Al.sub.2 O.sub.3 water vapor sensor which is compatible with conventional radiosondes. The circuit, together with the sensor, permits measurement of water vapor concentrations in the atmosphere and, more particularly, is the stratosphere.
Current radiosondes use the "carbon-element hygristor" which is inoperable at ambient temperatures below about -35.degree. to -40.degree. C. The Al.sub.2 O.sub.3 sensor is not temperature limited in this way. Presently known and commercially used measuring circuitry is both compatible with known radiosonde telemetry systems and too expensive for semi-routine use in a radiosonde whose recovery is in doubt.